Physics

Electromagnetic Waves and Spectrum

670 Questions

Electromagnetic waves and spectrum questions test a candidate's understanding of radiation frequencies, wavelengths, and the properties of different rays like infrared, ultraviolet, and visible light. Concepts also cover practical applications in astronomy and the fundamental speed of light calculations. This physics topic appears regularly in general science sections of major competitive examinations.

UV rays propertiesElectromagnetic radiation frequencyWavelength identificationSpeed of light calculationsBlack body radiation

Electromagnetic Waves and Spectrum Questions

Multiple choice standardized measurement measurement of physical quantities need of unit for measurement measurements and experimentation physics

The wavelength of light is usually expressed in

  1. Micron

  2. Fermi

  3. Nanometer

  4. Angstrom
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Angstrom $(\dot A)$ unit of length used chiefly in measuring wavelengths of light, equal to $10^{-10}$ metre, or $0.1$ nanometer.  It is used to express wavelengths of visible light, ultraviolet (UV) light, X rays, and gamma rays.

Multiple choice standardized measurement measurement of physical quantities need of unit for measurement measurements and experimentation physics

The wavelength of light is expressed in

  1. Metre

  2. Micron

  3. Light year

  4. Angstrom
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Angstrom $(\dot A)$ unit of length used chiefly in measuring wavelengths of light, equal to $10^{-10}$ metre, or $0.1$ nanometer.  It is used to express wavelengths of visible light, ultraviolet (UV) light, X rays, and gamma rays.

Multiple choice physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

The electric field of an electromagnetic wave traveling through the vacuum is given by the equation $E=E _0\ sin (Kx-\omega t).$ The quantity that is independent of wavelength is:

  1. $k\omega$
  2. $\dfrac{k}{\omega}$
  3. $k^2\omega$
  4. $\omega$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

To find: The quantity that is independent of the wavelength.


The angular frequency $\omega$ is given by:
$\omega \, = \, 2\pi \nu$
The frequency of a wave varies with the wavelength. So, angular frequency is dependent on wavelength.

The quantity $k$ is defined as the wavenumber and it is given by:
$k = \dfrac{2\pi}{\lambda}$
It shows that $k$ is dependent on wavelength.


The value of $\dfrac{k}{\omega}$ can be obtained as:
$\dfrac {k}{\omega} \, = \, \dfrac{2\pi / \lambda}{2\pi \nu}\\implies \, \dfrac{1}{\nu \lambda} \, = \, \dfrac{1}{c}\,\,\ \ \ \ \ \ \ \ \ \ \ \ \  (\because \, c \, = \, \nu \lambda)$
where c is the speed of electromagnetic wave in vacuum. It is a constant whose value is $3 \, \times \, 10^8 \, ms^{-1}$.

So, option $(B)$ is correct.

Multiple choice physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

According to Maxwell's equation, the velocity of light in any medium is expressed as

  1. $\displaystyle\frac{1}{\sqrt{\mu _0\varepsilon _o}}$
  2. $\displaystyle\frac{1}{\sqrt{\mu\varepsilon}}$
  3. $\displaystyle\sqrt{\frac{\mu}{\varepsilon}}$
  4. $\displaystyle\sqrt{\frac{\mu _0}{\varepsilon}}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Velocity of light in a medium,

$\displaystyle c=\frac{1}{\sqrt{\mu _0\varepsilon _o\mu _r\varepsilon _r}}=\frac{1}{\sqrt{\mu\varepsilon}}$

Multiple choice physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

Wavelength of light in different media are proportional to:

  1. speed of light in that medium

  2. Amplitude of light in that medium

  3. frequency of light in that mrdium

  4. Nove of above
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The speed of light in a medium is given by v = f * lambda. Since frequency (f) remains constant when light moves between media, the wavelength (lambda) is directly proportional to the speed of light (v) in that medium.

Multiple choice physics option a: relativity maxwell's equations the nature of light introduction to electromagnetic waves

A plane electromagnetic wave with an intensity of $200 W/m^2$ is incident normal to a flat plate of radius 30 cm. If the plate absorbs $60%$ and reflect $40%$ of the incident radiation, what is the momentum transferred to it in 5 min?

  1. $1.7 \times 10^{-3} kg ms^{-1}$
  2. $2.7 \times 10^{-4} kg ms^{-1}$
  3. $3.7 \times 10^{-4} kg ms^{-1}$
  4. $3.7 \times 10^{-3} kg ms^{-1}$
Reveal answer Fill a bubble to check yourself
A Correct answer
Multiple choice chemistry introduction to analytical chemistry interpreting a balanced chemical reaction percentage yield stoichiometric calculations

One mole of photons is known as one Einstein of radiation. According to Stark-Einstein law of photochemical equivalence, one mole of reactant absorbs one Einstien of energy. For a photochemical reaction, a term called quantum yield is defined as:
Quantum yield $ (\phi) = \dfrac {No. \,of \,moles \,of \,reactant \,converted} {No. \,of \,Einstein \,absorbed} $
The correct statement(s) is/are:

  1. for a chain reaction $\phi _{gas} >> \phi _{solution}$
  2. in a photochemical chain reaction $\phi >> 1$
  3. in a photochemical chain reaction $\phi << 1$
  4. for a chain reaction $\phi _{gas} << \phi _{solution}$
Reveal answer Fill a bubble to check yourself
A,B Correct answer
Explanation
$Quantum \ Yield= \cfrac {Number \ of \ moles \ of \ reactant \ converted}{Number \ of \ einstein \ absorbed}$
An einstein of radiation $=$ one mole of photons
For a chain reaction, $\phi _{gas} >> \phi _{solution}$
and ln photochemical chain reaction $\phi >>1$
Multiple choice physics communication system commonly used terms in electronic communication system elements of communication system electromagnetic waves and communication system

Generation, propagation and detection of electromagnetic waves is the basis of

  1. lasers

  2. reactors

  3. radio and television

  4. computer
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The communication and broadcasting following the base on generation, propagation, and detection of electromagnetic waves.

The electromagnetic spectrum describes a different range of electromagnetic waves. These EM waves are a special type of wave that can travel without a medium.

Electromagnetic waves are named like this due to the fact that they have both an electric and a magnetic component. In a vacuum, EM waves always travel at the same speed i.e. the speed of light. So, other EM waves besides light are infrared, ultraviolet, radio waves, and microwaves. Therefore radio and television both are based on EM wave properties. Other options like lasers, reactors, and computers are not guided by EM waves.

Thus the correct option is C. 

Multiple choice physics communication system elements of a communication system elements of communication system electromagnetic waves and communication system

In sky wave communication which of the frequency will be reflected back by the ionospheric layer having electron density $4 \times 10 ^ { 6 } / \mathrm { m } ^ { 3 }$

  1. $19 \times 10 ^ { 3 } \mathrm { Hz }$
  2. $9 \times 10 ^ { 3 } H z$
  3. $31 \times 10 ^ { 3 } \mathrm { Hz }$
  4. $20 \times 10 ^ { 3 } \mathrm { Hz }$
Reveal answer Fill a bubble to check yourself
B Correct answer
Multiple choice physics communication system elements of a communication system elements of communication system electromagnetic waves and communication system

An antenna is a device that converts

  1. electromagnetic energy into radio frequency

    signal

  2. radio frequency signal into electromagnetic

    energy

  3. guided electromagnetic waves into free space

    electromagnetic waves

  4. free space electromagnetic waves into guided

    electromagnetic waves
Reveal answer Fill a bubble to check yourself
C,D Correct answer
Explanation

$Answer:-$ C ,D

An antenna (plural antennae or antennas), or aerial, is an electrical device which converts electric power (guided electromagnetic waves) into radio waves(free spaceelectromagnetic waves), and vice versa. It is usually used with a radio transmitter or radio receiver.

Multiple choice physics option a: relativity the nature of light speed of light and optical density introduction to light

Choose the correct answer from the alternatives given.
An electromagnetic wave of frequency $\nu= 3\ MHz$ passes from vacuum  into a dielectric medium with permittivity $\varepsilon= 4$. Then

  1. wavelength and frequency both become half.

  2. wavelength is doubled and frequency remains unchanged.

  3. wavelength and frequency both remain unchanged.

  4. wavelength is halved and frequency remains unchanged.
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation
Given : frequency $v =3 MHz=3\times10^6Hz$, relative permitivity $\varepsilon _r = 4$
Here the frequency of electromagnetic wave remains unchanged but the wavelength of electromagnetic wave changes when it passes from one medium to another.
The refractive index is the square root of permeability and permittivity product. 
For formula,
$c=\dfrac 1{\sqrt {\mu _0\varepsilon _0}}\\\implies c\propto \dfrac1{\sqrt{\varepsilon _0}}$
Similarly,
$v\propto\dfrac1{\sqrt{\varepsilon}}$
Therefore,
$\dfrac cv=\sqrt{\dfrac {\varepsilon}{\varepsilon _0}}=\sqrt{\dfrac 41}=2........(i)$
But
$\dfrac cv=\dfrac {\nu\lambda}{\nu\lambda'}\\\implies \dfrac cv=\dfrac{\lambda}{\lambda'}\\\implies 2=\dfrac{\lambda}{\lambda'}\\\implies \lambda'=\dfrac \lambda2$
Hence wavelength is halved.